1. Field of the Invention
The field of the invention is that of optical fiber cooling tubes, i.e. cooling tubes for cooling optical fibers during drawing thereof. An optical fiber is drawn from a preform placed in a furnace. At the exit from the furnace, the optical fiber is at a very high temperature and cannot be cooled quickly before its temperature has fallen to around 600° C. Also, the optical fiber is yet to receive a cladding, which cannot be applied until the temperature of the optical fiber has fallen below 100° C., typically to around 70° C. The cooling tube is usually located between the exit from the furnace and the device for applying the cladding to the optical fiber, and its function is to cool considerably, typically from 500° C. to less than 100° C., over a short distance, typically less than 4 m or even 2 m, an optical fiber whose drawing speed can be high, for example 1 500 meters per minute (mpm) or more. During drawing, because of the short cooling distance and the high drawing speed, the time for which the optical fiber remains in the cooling tube is very short, typically of the order of a few tenths of a second, which implies the use of particularly efficient cooling tubes. The cooling tubes operate by circulating a gas with a higher thermal conductivity than air, typically helium, although other gases can be envisaged. The higher the drawing rate, the higher the flowrate of helium necessary to achieve a given cooling efficiency.
2. Description of the Prior Art
A first prior art cooling tube for use at low drawing speeds consists of an elongate cavity surrounding the optical fiber over a distance of 3 m to 4 m and at the entry end of which helium is injected. The consumption of helium is reasonable, not exceeding a few tens of liters per minute. This kind of tube is long, however, and becomes inefficient as the drawing speed increases.
A second prior art cooling tube, described in U.S. Pat. No. 4,761,168, for example, for use at higher drawing speeds, has a large cavity studded with spikes intended to disrupt the boundary layer entrained with the optical fiber. During drawing, the optical fiber passes through the cooling tube at a particular speed and entrains with it a layer of gas in its immediate vicinity and moving at approximately the same speed; this layer of gas is referred to as the boundary layer and its extent depends on the drawing speed in particular. The boundary layer has the drawbacks of consisting mainly of air, whose thermal conductivity is lower than that of helium, and of not mixing much by diffusion with the remainder of the gas between the optical fiber and the walls of the cooling tube cavity, which greatly reduces the efficiency of heat exchange between the optical fiber and the injected helium. Nevertheless, for high drawing speeds, typically from 1 500 mpm to 1 800 mpm, cooling rapidly consumes a prohibitive quantity of helium, assuming that cooling can remain sufficiently efficient over a short cooling distance.
The invention proposes a solution based on a cooling tube of improved cooling efficiency that is particularly beneficial at high drawing speeds. The cooling tube has a structure such that helium can be injected repeatedly along the tube in a contraflow direction relative to the direction in which the optical fiber moves.